I've seen numbers like 50 MHz vs 1 GHz, so a factor of 20. But that's just raw clock speed. Here's a very interesting talk by a person named Henry Wong who built an out-of-order instruction-level parallel x86 implementation with a soft core, which compensates somewhat for the much slower clock speed. This is a pretty heroic piece of FPGA wizardry:

https://hackaday.com/2019/06/22/fpga-soft-cpu-is-superscalar...